A wide variety of processes and techniques have been developed to separate and recover hydrogen from a multi-component gas stream.
U.S. Pat. No. 3,359,744 discloses a method for removing hydrocarbons from a crude hydrogen stream wherein a portion of a purified hydrogen stream is mixed with condensed and separated hydrocarbons and the mixture is used to refrigerate and condense the incoming crude hydrogen to obtain increased purity of the product hydrogen. This system also utilizes either product hydrogen or separated hydrocarbons to regenerate and cool a plurality of switching adsorbers which remove water and other impurities prior to condensation of the crude hydrogen stream.
U.S. Pat. No. 3,864,465 disclosed a method of purifying hydrogen off-gas to at least 90% H.sub.2 by initially contacting the off-gas with a bed of fluidized iron oxides at temperatures between 160.degree. C. and 310.degree. C. and pressures between 100 psi and 1,000 psi. The H.sub.2 -containing gas from the fluidized iron oxide bed is then passed through a cryogenic separator to remove impurities, which are mostly methane, to produce a purified hydrogen stream.
U.S. Pat No. 4,242,875 describes a process for the cryogenic purification of industrial by-product gas streams which contain hydrogen in recoverable amounts in which a second, separate feed stream is employed to enhance the refrigeration of the purification system and permit the recovery of an increased amount of hydrogen of the desired purity. Providing a second, separate feed stream allows by-product hydrogen streams to be used which contain non-readily condensable impurities with boiling points below that of methane, such as nitrogen or helium, which are detrimental to a hydrogen product utilized in hydrocracking or hydrotreating processes.
A membrane separation technique is described in U.S. Pat. No. 3,975,170. The method involves passing a fluid mixture whose hydrogen concentration is to be controlled through one chamber of a diffusion cell separated into two chambers by a hydrogen permeable membrane. A gradient of hydrogen partial pressure is then maintained across the membrane at a level sufficient to cause diffusion of hydrogen through the membrane to maintain the concentration of hydrogen in the fluid mixture at a predetermined level. The hydrogen concentration of the fluid may be controlled to some positive value by maintaining a second, fluid-containing hydrogen gas in the other chamber of the diffusion cell, or the hydrogen concentration in the fluid may be controlled to a value approximating zero hydrogen concentration by maintaining a near vacuum in the other chamber of the diffusion cell.